<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        * {
            padding: 0;
            margin: 0;
            border: 0;
        }

        .clearFloat{
            width: 100%;
            overflow: hidden;
        }

        /*.clearFloat::after {*/
        /*    content: none;*/
        /*    clear: both;*/
        /*    display: block;*/
        /*    height: 0;*/
        /*    visibility: hidden;*/
        /*}*/

        .rightSide {
            float: right;
            width: 200px;
            background-color: #FF0000;

            /*margin-left: 10px;*/
            min-height: 500px;
        }

        .leftSide {
            text-align: right;
            background-color: blue;

            margin-right: 10px;
            min-height: 500px;
        }

    </style>
</head>
<body>
<div class="clearFloat">
    <div class="rightSide">右侧固定</div>
    <div class="leftSide">左侧自适应</div>
</div>
</body>
</html>